The present invention relates to a method of driving an inkjet recording head and an inkjet recording apparatus, and specifically, to a driving technique for driving an inkjet recording head for the recording of characters and images by the ejection of minute ink droplets from an ink nozzle in an inkjet recording apparatus.
As a conventional inkjet recording system, a drop-on-demand type inkjet system is known whereby an electro-mechanical transducer such as a piezoelectric actuator is used to cause a pressure wave (acoustic wave) to be generated in a pressure chamber filled with a liquid ink, so that the pressure wave ejects an ink droplet from a nozzle coupled with the pressure chamber. Such inkjet recording method using the drop-on-demand type inkjet system is disclosed in Japanese Patent Publication No. Sho. 53-12138, for example. An example of the structure of the inkjet recording head of this type is shown in FIG. 22.
Referring to FIG. 22, a pressure chamber 61 is connected with a nozzle 62 for the ejection of ink and an ink supply path 64 for guiding ink from an ink reservoir (not shown) through a common ink chamber 63. A vibrating plate 65 is mounted on the bottom surface of the pressure chamber.
When an ink droplet is to be ejected, a piezoelectric actuator 66 mounted outside the pressure chamber 61 operates to displace the vibrating plate 65, whereby the volume within the pressure chamber 61 is changed and thus a pressure wave is generated therein. This pressure wave causes a part of the ink filled in the pressure chamber 61 to be ejected through the nozzle 62 as a flying ink droplet 67. The flying ink droplet lands on a recording medium such as a recording paper and forms a recorded dot thereon. Such formation of recorded dots are repeated on the basis of image data, thereby recording characters or images on the recording paper.
In order to achieve a high image quality in this type of inkjet recording head, it is necessary to minimize the diameter of the ejected ink droplet (droplet diameter). Specifically, in order to obtain a smooth image with little graininess, the recording dot (pixel) formed on the recording paper must be made as small as possible. For this reason, the diameter of the ink droplet ejected must be minimized in size. Generally, the graininess of the image decreases greatly as the dot diameter becomes 40 xcexcm or less. As the dot diameter becomes 30 xcexcm or less, it becomes so difficult to visually recognize the individual dots even in the highlight portion of the image that the image quality improves greatly.
The relationship between the ink droplet diameter and the dot diameter depends on the rate of flight of the ink droplet (droplet velocity), physical properties of the ink (viscosity, surface tension), the type of the recording paper, and so on. Normally, the dot diameter is about twice the size of the ink droplet diameter. Accordingly, in order to obtain a dot diameter of 30 xcexcm or less, the ink droplet diameter must be set at 15 xcexcm or less. In the present description, the diameter of the ink droplet (droplet diameter) refers to the diameter of a spherical droplet substituting the total amount of ink (including the satellites) ejected in a single act of ejection.
The most effective way of minimizing the ink droplet diameter is to reduce the nozzle diameter. Practically, however, the nozzle diameter cannot be reduced to less than about 25 xcexcm, given technical difficulties in the manufacture and the fact that as the nozzle diameter is reduced, the nozzle tends to be clogged. Accordingly, it is impossible to obtain an ink diameter on the order of 15 xcexcm solely by decreasing the nozzle diameter. To solve this problem, it is known to reduce the droplet diameter of the ejected ink droplet by way of the driving method employed, and some effective methods are proposed.
As one such example, Japanese Patent Laid-open Publication No. Sho. 55-17589 discloses a meniscus control technique whereby the pressure chamber is once expanded immediately before ejection, and then an ink droplet is ejected when the ink meniscus at the nozzle opening is drawn towards the pressure chamber. FIG. 23 shows an example of the driving waveform for driving the piezoelectric actuator using this technique. In the present description, the relationship between the driving voltage and the piezoelectric actuator operation is such that as the driving voltage increases, the volume of the pressure chamber decreases and, conversely, as the driving voltage decreases, the volume of the pressure chamber increases. Generally, the polarities are often reversed depending on the structure of the piezoelectric actuator and the direction of polarization of the piezoelectric element.
Referring to the driving waveform shown in FIG. 23, a voltage fall 71 from V1 to zero volt expands the volume of the pressure chamber. A subsequent voltage rise 71 from zero volt to V2 compresses the volume of the pressure chamber to thereby eject an ink droplet. The interval of each of the fall time t1 and rise time t2 is generally on the order of 2-10 xcexcs, which is longer than an inherent period Ta of the conventional piezoelectric actuator.
FIGS. 25(a) to (d) illustrate the movement of the ink meniscus at the nozzle opening portion upon application of the driving waveform of FIG. 23. The ink meniscus has a flat upper portion during the initial state (FIG. 25(a)). As the pressure chamber is expanded immediately before the ejection, the top portion of the ink meniscus assumes a concave shape, as shown in FIG. 25(b). As the pressure chamber is compressed by voltage rise 71 when there is such a concave ink meniscus, a thin liquid column 83 is formed in the center of the ink meniscus as shown in FIG. 25(c). This is followed by the formation of an ink droplet 84 as the tip of the liquid column is separated (FIG. 25(d)). The ink droplet diameter is substantially equal to the thickness of the liquid column thus formed and is smaller than the nozzle diameter. Thus it is possible to eject an ink droplet with a smaller diameter than the nozzle diameter by using such driving method.
As described above, the meniscus control system enables the ejection of an ink droplet with a smaller diameter than the nozzle diameter. However, when such driving waveform as shown in FIG. 23 is used, the smallest diameter of the droplet that could actually be obtained was about 25 xcexcm, which is still not good enough to satisfy the need for higher image quality.
FIG. 24 shows another driving waveform as a driving means for enabling the ejection of a smaller droplet. In this waveform shown in FIG. 24, a voltage fall 73 draws the ink meniscus immediately prior to the ejection. A subsequent voltage rise 74 compresses the volume of the pressure chamber and thereby causes a liquid column to be formed. A voltage fall 75 separates a droplet from the tip of the liquid column at an early period. A voltage rise 76 suppresses the reverberations of the pressure wave remaining after the ejection of the ink droplet. In other words, the driving waveform of FIG. 24 is based on the conventional meniscus control system in which a pressure wave control is incorporated for the early separation of the ink droplet and for the suppression of the reverberations. This arrangement allows an ink droplet with a droplet diameter on the order of 20 xcexcm to be ejected in a stable manner.
However, it was still difficult to eject an ink droplet with an ink diameter of 20 xcexcm or less easily even by using this improved driving waveform, and particularly an ink diameter of 15 xcexcm or less was impossible. Thus, there was no driving method that could achieve the ink diameter of 15 xcexcm or less, which was required for image quality reasons. One of the biggest reasons for this was that in the conventional inkjet recording head, the ink droplet ejection was carried out by the pressure wave that was governed by the acoustic capacity of the pressure chamber. This reason will be explained in detail below.
FIG. 26 shows the result of observation of velocity changes in the ink meniscus (particle velocity change) by a laser Doppler meter, the changes being caused when a driving waveform of FIG. 24 is applied to the piezoelectric actuator. As shown in the figure, the ink meniscus vibrates due to the pressure wave generated in the pressure chamber. In the example of FIG. 26, the inherent period Tc of the pressure wave is 13 xcexcs, and pressure waves generated at the respective nodes of the driving waveform are superposed, resulting in a complex velocity change in the ink meniscus.
The volume of the ejected ink droplet can be thought of as substantially proportional to the product of a shaded area defined by the initial positive half-cycle of the pressure wave of FIG. 26 and the area of the nozzle opening. Namely, an estimate of the droplet diameter (drop volume) on the assumption that the ink is ejected from the nozzle with a positive rate (velocity in the direction out of the nozzle) and flies as an ink droplet corresponds well with an actually measured droplet diameter (drop volume). Although when the meniscus control system is used, a liquid column which is thinner than the nozzle diameter is formed and therefore the effective nozzle opening area decreases, the relationship where the ink droplet volume is substantially proportional to the shaded area of FIG. 26 is still valid. Accordingly, inorder to reduce the droplet diameter (drop volume), it is important to reduce the area of the above-mentioned shaded portion.
There are roughly two ways for the reduction of the shaded portion area. One sets the amplitude of the particle velocity small, as shown in FIG. 27. The other sets the period of the particle velocity vibration short, as shown in FIG. 28. The former method, by which the amplitude of the particle velocity is set small, is difficult to implement in actual applications. This is because the drop velocity is substantially proportional to the average particle velocity of the shaded portion, and so if the amplitude of the particle velocity is set small, the flying velocity (drop velocity) of the ink droplet drops significantly, which poses a problem in image recording.
Accordingly, in order to perform a minute-drop ejection, the inherent period of the pressure wave must be set very small as shown in FIG. 28. Specifically, in order to eject an ink droplet with a droplet diameter of 15 xcexcm at a drop velocity of 6 m/s, the inherent period of the pressure wave must be set on the order of 3 to 5 xcexcs.
However, it was very difficult to set the inherent period of the pressure wave at such small values in the conventional inkjet recording head. This was because of the fact that in order to obtain the inherent period on the order of 3 to 5 xcexcs, the volume of the pressure chamber must be set very small and at the same time the rigidity of the walls forming the pressure chamber must be very high, as will be described later. Those measures, however, are difficult to realize in the conventional head manufacturing method where the pressure chamber is constructed by stacking and bonding perforated board materials.
Even if the above-mentioned conditions are met, the reduction in the limit ejection frequency of the ink droplet cannot be avoided. Specifically, while it is necessary to set the volume of the pressure chamber small in order to shorten the inherent period of the pressure wave, a certain area must be secured for the actuator unit for the application of displacements by the piezoelectric actuator, which necessarily results in the pressure chamber having a flat shape. As a result, the flow-path resistance of the pressure chamber significantly increases, which in turn lengthens the refill time (the time for the returning of the ink meniscus after ejection), thereby making it difficult to repeat the ejection at a high frequency.
As explained above, the conventional inkjet recording head had the disadvantage that it is unable to eject an ink droplet with such a droplet diameter as required for the significant improvement of the image quality, namely a minute ink droplet with a droplet diameter on the order of 15 xcexcm.
An object of the present invention is to provide a method of driving an inkjet recording head which is capable of ejecting an ink droplet with a droplet diameter of 15 xcexcm or less without adversely affecting the ejection property in the high-frequency region and without requiring a specialized head manufacturing technology, and to provide an inkjet recording apparatus using such driving method.
Another object of the present invention is to enable both high-quality and high-speed recording by ensuring a wide range of droplet diameter modulation when performing a grayscale recording by modulating the droplet diameter of the ejected ink droplet in multiple levels.
In order to achieve those objects, the present invention is directed to a method of driving an inkjet recording head having a pressure chamber filled with a liquid ink, said pressure chamber including an ink supply port for supplying the liquid ink and an ink nozzle for ejecting said ink in the form of at least one ink droplet, and an electro-mechanical transducer disposed such that a pressure wave is generated in said pressure chamber by applying a driving voltage in order to eject the ink droplet via said ink nozzle, said transducer having an inherent vibration period Ta, said method characterized in that:
said driving voltage has a first driving voltage waveform, said first driving voltage waveform including consecutively a first waveform portion having a first time length t1 for contracting a volume of said pressure chamber and a second waveform portion having a second time length t2 for expanding the volume of said pressure chamber, said first and second time lengths t1 and t2 being set equal to or longer than the inherent vibration period Ta of said electro-mechanical transducer.
An inkjet recording apparatus according to the present invention includes: an inkjet recording head including a pressure chamber having an ink supply port for supplying a liquid ink and an ink nozzle for ejecting the ink as at least one ink droplet, the pressure chamber being filled with liquid ink, and an electro-mechanical transducer disposed such that the ink droplet is ejected from the ink nozzle by the generation of a pressure wave in the pressure chamber by application of a driving voltage, the transducer having an inherent vibrating period Ta; and
a driving waveform generating circuit for generating one or more driving waveforms for the driving voltage to be applied to the electro-mechanical transducer, wherein:
the driving waveform includes a first waveform portion having a first time length for the compression of the volume of the pressure chamber and a second waveform portion having a second time length for the expansion of the volume of the pressure chamber, the first and second time lengths being set equal to or longer than the inherent vibrating period Ta of the electro-mechanical transducer.
In accordance with the method of driving the inkjet recording head and the inkjet recording apparatus according to the invention, the electro-mechanical transducer element is actuated by a driving waveform having a rise time and a fall time which are shorter than the inherent vibrating period of the electro-mechanical transducer element, whereby a minute ink droplet having a diameter of 15 xcexcm or less can be ejected from the ink nozzle and therefore the printing precision can be improved.